When changing engine oil, you have probably been asked about which weight or grade you prefer. The property called "viscosity" relates to the internal friction of a fluid. A low viscosity will flow more easily while a high viscosity is associated with a heavier fluid meant for specific driving conditions. The system of numbers assigned has to do with which weather conditions you should use the product in. It is important for you to know if you need to switch viscosities when getting an oil change. If you're from out of the area, an oil change specialist cannot immediately determine what kind of conditions you drive in on sight of your vehicle.
The number assigned to types by the Society of Automotive Engineers (SAE) has nothing to do with quality. It is simply an indicator for what type of weather it was designed. For example, SAE 10 is recommended for cold weather operation while SAE 30 is for those vehicle used in warm weather climates. There are single viscosities, such as the aforementioned, and multi-viscosity numbers. Sometimes a single viscosity is referred to as a straight weight, and is used in areas where the temperature is consistent. Multi-viscosity oils are used in areas that experience extreme seasonal changes.
The letter W that often accompanies the number assigned to the weight stands for "winter grade." The absence of a W by a number indicates a warmer weather grade. SAE 5W-30 therefore is a multi-weight that is suggested for vehicles that are operated in extreme winters and mild to normal warmer weather. 20W-40 is for mild winters but extreme summers.
Most vehicles purchased on a car lot will be equipped with the weight that is suggested for the weather conditions in which the car is sold, and it is important to remember that the weight has nothing to do with the type of car; it has only to do with the weather condition in which the car is driven. If you purchase a car from a lot in Texas but you live in Alaska, you will need an oil change to address the change in necessary viscosity once returning home to Alaska.
It is not the responsibility of nor is it possible for a specialist to predict what type is best for you and your driving conditions, so make sure to initiate the conversation with the technician with each maintenance check. You may even need to choose and change to a different weight before your traditional interval has been met.